The present invention relates to the art of power driven threading machines and, more particularly, to chucks for gripping a plastic coated workpiece and rotating the workpiece during cutting, reaming and/or threading thereof.
Power driven thread cutting machines are well known and basically comprise a rotatable chuck assembly for supporting and rotating a workpiece, a tool carriage supporting tools for performing work on the rotating workpiece, and a drive unit for rotating the chuck assembly and thus the workpiece. The chuck assembly generally includes a tubular spindle having an axis of rotation and axially opposite ends, and a plurality of workpiece engaging jaws pivotally supported at the opposite ends of the spindle for displacement radially inwardly and outwardly of the spindle axis between closed and open positions with respect to a workpiece introduced through the spindle. The jaws on the spindle provide for a workpiece to be gripped at axially spaced apart locations along the length thereof, and in the closed positions of the jaws the workpiece is held in a centered position coaxial with the spindle. The spindle and jaws are rotatable as a unit to rotate the workpiece relative to the tool carriage which usually carries a cutting tool, a reaming tool and a thread cutting die head. When the jaws are in the open positions thereof, the workpiece is released and is adapted to rest on the inner periphery of the tubular spindle. The spindle diameter is such that the chuck assembly is adapted to grip and support workpieces having different diameters up to a given maximum diameter which is less than the inner diameter of the spindle. A drive unit is provided for the chuck assembly and, in connection with initiating operation of the threading machine, the jaws are generally pivoted relative to the spindle and into engagement with a workpiece positioned in the spindle, after which the workpiece and chuck assembly are driven together by the drive unit through the jaws. Upon completion of the cutting, reaming and/or threading operation, the direction of the drive is reversed, whereby the jaws pivot outwardly of the spindle axis to release the workpiece.
Typically, the workpiece engaging portions of the jaws of such chucks are serrated to promote gripping interengagement with the workpiece so as to prevent slippage of the workpiece relative to the chuck assembly during a threading operation. Such serrated workpiece engaging jaws are shown, for example, in U.S. Pat. Nos. 2,890,888 to Damijonaitis; 2,980,434 to Hoffman; and, 3,413,667 to Behnke. As will be seen from the latter prior art, three or four such serrated jaw members are equally spaced apart circumferentially of the chuck axis and, upon gripping interengagement with the workpiece, the teeth penetrate the surface of the workpiece sufficiently to preclude slippage of the workpiece relative thereto during the performance of threading or other work on the workpiece. While such jaw configurations work satisfactorily in connection with gripping and rotating a metal workpiece, a number of problems are encountered in connection with efforts to use such jaw configurations to grip and rotate a plastic coated workpiece, such as a plastic coated pipe for example. Construction codes in many locations require the use of plastic coated pipe, when pipe is buried under ground, to protect the metal pipe from deterioration. Such a plastic coating on metal pipe is typically about 1/16" thick, and the plastic materials differ in degrees of hardness and are applied about the pipe by different methods which accordingly provide different bonding strengths between the pipe and coating. In connection with efforts to grip and rotate a plastic coated pipe through the use of workpiece gripping jaws heretofore known, it has been necessary to penetrate the plastic coating in order for the teeth of the jaws to grip the metal pipe to preclude slippage of the workpiece relative to the jaws during a cutting, reaming and/or threading operation. More particularly in this respect, the gripping jaw arrangements heretofore provided have a small area of contact with the outer surface of the workpiece and the plastic does not have sufficient strength to withstand the torsional load on the plastic coating during working. Therefore, the pipe slips relative to the gripping jaws and the jaws groove the plastic circumferentially all the way down to the outer surface of the metal pipe therebeneath. If the bond between the plastic and metal pipe is weak, partial penetration of the plastic in a small area of contact with the jaws can result in the plastic being torn from the pipe. Obviously, neither of these conditions is acceptable and require either repair of the plastic or replacement of the pipe being worked upon, both of which are costly from the standpoint of material and time. While such destruction of the plastic coating can be avoided by full penetration of the teeth of the gripping jaws into the metal of the pipe, such full penetration exposes the pipe and this is not acceptable by code and requires repair of the pipe prior to its use to close the openings caused by such penetration. This again is time consuming and expensive. Moreover, even if the teeth of the workpiece gripping jaws are of a sufficient length to fully penetrate the plastic coating, there is the possibility that the undesirable slippage and destruction of the plastic will occur upon driving engagement of the jaws with the plastic coating before full penetration is achieved.